A number of transmissive mode displays using smectic C* phase liquid crystal have been proposed. Molecules of a liquid crystal having a smectic C* phase are elongated and chiral. Smectic C* phases are characterized in that the molecules align in layers with their molecular directors oblique with respect to the layers. The tilt directions are twisted with respect to one another, so that the directors trace the surface of a cone. Smectic C* materials are "ferroelectric" in that the electrical polarization of the liquid crystal may be realigned with an electric field and remains stable in thin films once the field is removed.
"Surface-stabilized ferroelectric liquid crystal" ["SSFLC"] displays make use of thin films of smectic C* liquid crystal confined between substrates. In one proposed device of this type, the liquid crystal is confined between substrates which are placed between crossed optical polarizers. Preferably, the liquid crystal is aligned in a so-called "bookshelf" texture in which the molecules are arranged in layers perpendicular to the inner surfaces of the substrates and the molecular directors are approximately parallel along a line normal to the layers. If the film is sufficiently thin, the bookshelf texture should be stable when an electric field is removed. The material may be switched between two stable bookshelf textures by generating external electric fields across the liquid crystal normal to the inner surfaces of the substrates. The field useful for switching the material from one texture to the other is opposite in polarity to the field useful for switching back to the first of the textures. The two stable bookshelf textures are similar to each other in that the molecular directors are aligned homogeneously, but differ in that the molecular directors in the two textures form mirror images about a plane normal to the layers and to the inner surfaces of the substrates.
SSFLC displays modulate light by controlling the polarization direction of linearly polarized light transmitted through the liquid crystal. Due to the birefringence of the liquid crystal, the component of light incident on the liquid crystal is decomposed into two components of different speeds which change the polarization dependent on the thickness of the liquid crystal material. In an SSFLC display, the upstream polarizer is aligned so that the polarization direction of light incident on the liquid crystal is parallel or perpendicular to the molecular directors in a first of the two stable bookshelf textures; in this texture, the incident polarized light is transmitted without change of the polarization direction by the liquid crystal and is consequently blocked by the downstream analyzer. In addition, the thickness of the liquid crystal layer is selected to effect a 90.degree. rotation of the polarization direction of the polarized light incident on the liquid crystal when the liquid crystal is in the second of the two stable bookshelf textures, so that the polarized light transmitted by the liquid crystal in this second texture is transmitted by the analyzer. Consequently, the SSFLC display is dark when the liquid crystal is in the first of the stable bookshelf textures and bright when the liquid crystal is in the second of the two bookshelf textures.
Among the advantages attributed to displays using smectic C* liquid crystal are fast switching times, low switching voltage and high contrast compared to commercially available liquid crystal displays such as twisted nematic displays. In particular, displays using ferroelectric smectic C* liquid crystal may be capable of switching times on the order of microseconds.
One disadvantage of displays using smectic C* liquid crystals is the difficulty of obtaining stable bookshelf alignment of the liquid crystal. The layers of smectic C* materials confined between substrates have a tendency to undulate or twist, producing defects which reduce the contrast. The liquid crystal is usually aligned on cooling from a high temperature using a surface coating which promotes homogeneous alignment of the molecules near the coating. This method gives a good bookshelf texture at the high temperature limit of the smectic range (often a smectic A phase). Due to the temperature dependence of the layer thickness, however, the layers generally form defects on further cooling. Quasi-bookshelf textures can be obtained by the use of certain napthalene derivatives, for which the layer tilt is only about 5.degree. at room temperature. Furthermore, the use of surface coatings which promote homogeneous alignment in order to induce a bookshelf texture significantly increases the switching voltage required to switch between stable textures. Despite intense research over the past decade, there remains a need for method of aligning smectic C* liquid crystal in a stable bookshelf alignment capable of low voltage switching. In particular, there is a need for a method to align the films in the finished display.